Control circuit



Dec. 19, 1950 J. I... BOYER ETAL 2,534,754

CONTROL CIRCUIT Filed Oct. 51, 1945 2 Sheets-Sheet 1,

WITNESSES: INVENTOR5 j'ohn LBayer and 9- Mmgm .Epal a/a.

ATTORNE Dec. 19, 1950 J. L. BOYER ElAL 2,534,754

CONTROL CIRCUIT Filed Oct. 51, 1945 2 Sheets-Sheet 2 |NVENTOR5 John A. Boygr and h/g/Y/l'am ffako /a,

ATTORNE Patented Dec. 19, 1950 UNITED STATES PATENT OFFICE CONTROL CIRCUIT Application October 31, 1945, Serial No. 625,908

12 Claims. (01. 321-56) This invention relates to an electronic control circuit, and it has particular relation to a circuit for use in supplying a load with single-phase current from a source of polyphase alternating voltage.

In the usual resistance weldin machine cir- Quit, the welding transformer is supplied with a single-phase current taken from a single phase of the supply system. Since the supply system is ordinarily a three-phase system, such an arrangement is disadvantageous in that an unbalanced load is taken from the three-phase supply system. In addition, the peak KVA demand is high and the power factor is very low on the supply phase.

More recent circuits have been developed in attempts to take a balanced load from the threephase power supply. In a typical circuit, current from the polyphase supply system is first rectified. The rectified current is then supplied alternately through different halves of a centertapped primary winding of the welding transformer, the current through one half of the primary winding being opposite in direction to that subsequently supplied through the other half. By this arrangement, a single-phase low frequency current output is obtained from the secondary winding of the welding transformer. Such a system has certain advantages including those attributable to a low frequency but it encounters commutation difliculties and has the disadvantage of requiring a center tap on the primary winding of the welding transformer. Consequently, the transformer copper is not utilized to the highest extent.

It is, accordingly, an object of our invention to provide a new and improved control system for use in supplying a single-phase current to a load from a source of polyphase alternating voltage.

A further object of our invention is to provide a new and improved control system for use in supplying a load with single-phase current of one frequency from a source of polyphase alternating voltage of a different frequency.

Another object of our invention is to provide a new and improved control system for use in supplying a relatively low frequency single-phase current to a welding transformer from a source of higher frequency polyphase alternating voltage.

A still further object of our invention is to provide a new and improved control system for use in supplying a very low power factor load with a single-phase low frequency current from a source of higher frequency polyphase a1ter= nating voltage.

In accordance with our invention, a pair of unidirectional electric valves are provided for each phase of the source voltage. Each pair of valves is connected in inverse parallel-circuit relation with each other and in series with the corresponding phase of the source and the load, which may be the primary winding of a welding transformer It follows that load current in one direction may be conducted only through a first valve of each pair and load current in the opposite direction may be conducted only through a second valve of each pair. Control means are then provided for causing the first valves of the pairs to be conductive to effect a load current in said one direction during a predetermined first time interval and said second valves to be conductive to effect a load current in the opposite direction during a predetermined second time interval following the first interval. Thus, a low frequency single-phase current is supplied through the load and the frequency may be varied by varyin the first and second time intervals.

The features of our invention which we consider novel are set forth with more particularity in the appended claims. The invention itself, however, together with additional objects and advantages thereof may be better understood from the following description of specific embodiments thereof when read in connection with the accompanying drawings in which:

Figure 1 is a circuit diagram of a control system embodying our invention; and

Fig. 2 is a circuit diagram of a preferred modification of the apparatus of Fig. 1.

As shown in Fig. 1, power is obtained from the three-phase supply lines 3, 4 and 5 through a supply transformer 6. The material 1 to be welded is engaged between two electrodes 8 and 9 connected across opposite ends of the secondary winding H) of the welding transformer l l with one end of the primary winding l2 being connected to the center point l3 of the starconnected secondary windings I00, 200 and 300 of the supply transformer 6. Each of these secondary windings, of course, is for a different phase voltage and for convenience in following the diagram, all elements and circuits relating directly to the phase voltage of the first secondary winding I00 bear reference numbers from to 200; all elements and circuits directly relating to the phase voltage of the second secondary winding 200 bear reference numbers from 200 to 300; and all elements and circuits relating directly to the phase voltage of the third secondary Winding 300 bear reference numbers from 300 to 400. In addition, those elements and circuits relating to the supply of current in one direction through the primary winding I2 of the weldin transformer I I bear odd reference numbers, while those relating to the supply of current in the opposite direction bear even reference numbers.

The outer end of the first secondary winding I is connected to the other end of the primary winding I2 of the welding transformer I I through a pair of inversely-connected unidirectional main valves IOI and I02, preferably discharge valves of the arc-like type such as ignitrons. The second secondary winding 200 is also connected to the other end of the primary winding I2 of the welding transformer II through another pair of unidirectional main valves 20I and 202, preferably discharge valves of the arc-like type, and the third secondary winding 300 is likewise connected to the other end of the primary winding I2 through still another pair of unidirectional main valves 30I and 302, preferably discharge valves of the arc-like type.

The anodes I03, 203 and 303 of main valves IOI, 20I and IBM are connected respectively to the outer ends of the secondary windings I00, 200 and 300 of the supply transformer 6 while the cathodes I05, 205 and 305 are connected respectively to the primary winding I2 of the welding transformer II. The anodes I04, 204 and 304 of main valves I02, 202 and 302 are connected respectively to the cathodes I05, 205 and 305 of main valves IOI, 20I and 30I while the cathodes I06, 206 and 306 of main valves I02, 202 and 302 are connected respectively to the anodes I03, 203 and 303 of main valves IOI, 20I and SM. Thus, it is evident that current in one direction through the primary winding I2 of the welding transformer II may only be conducted through main valves IOI, 20I and 3M, while current, in the opposite direction through the primary winding I2 may only be conducted through main valves I02, 202 and 302.

The ignition circuits for the main valves IOI, I02, 20I, 202, 30l and 302 are similar to each other and each extends from the corresponding one of cathodes I05, I06, 205, 206, 305 and 306 through a corresponding pair of series-connected rectifier units I01 and I09, I08 and H0, 201 and 209, 208 and 2I0, 301 and 309, 300 and 3I0, preferably of the dry type, to the corresponding ignition electrodes II I, I I2, 2I I, 2 I2, 3I I and 3I2. A corresponding secondary winding H3, H4, 2I3, 2I4, 3I3 and 3I4 of a correspond ing ignition transformer H5, H6, 2I5, 2H5, 3I5 and 3I6 is connected to supply voltage between the junction point intermediate the corresponding pair of rectifier units and the corresponding cathode.

The primary windings H1, H8. 2", 2I8, 3I1 and 3| 8 of the ignition transformers are each connected across an individual firing capacitor II9, I20, 2I9, 220, 3I9 and 320 through a corresponding electric discharge valve I21, I22, 22I, 222, 32I and 322, preferably of the arc-like type such as a thyratron, hereinafter designated the firing valve. The individual firing capacitors II9, I20, 219, 220, 3I9 and 320 are connected to be charged from the supply lines 3, 4 and 5 through a charging transformer having sixphase star-connected secondary windings I23, I24, 223, 224, 323 and 324. Each of the six secondary windings of the charging transformer I4 corresponds to an individual firing capacitor and is connected thereacross to charge that capacitor through a corresponding current limiting resistor I25, I26, 225, 226, 325 and 326 and a corresponding rectifier I21, I28, 221, 321 and 328 shunted by a high resistance resistor I29, I30, 229, 230, 329 and 330. The phase relations of the voltages across the secondary windings of the charging transformer I4 with respect to the phase voltages of the supply transformer 6 are such that charging of a firing capacitor is initiated approximately 60 electrical degrees ahead of the supply phase voltage for the main valve associated with that capacitor. As will appear later, this enables the firing capacitor to be fully charged prior to the instant of ignition of the corresponding main valve.

The control circuits of the firing valve I2I, I22, 22I, 222, 32I and 322 are similar to each other and each extends from the corresponding cathode I3I, I32, 23I, 232, 33I and 332 through a resistor I5 across which a negative biasing voltage is impressed by means of a rectifier unit I6 energized from the supply lines 3, 4 and 5. The control circuit continues from the resistor I5 to an intermediate tap I1 on a saturating reactor I 8 across the outer terminals of which is impressed the output of a square-wave voltage generator I9 through a linear reactor 40. The generator I9 is arranged so that the frequency of the square-wave voltage output corresponds to the frequency of the single-phase current to be supplied through the primary winding I2 of the welding transformer II.

From the intermediate tap I 1 on the saturating reactor I8, the control circuits of the firing valves then separate from each other. The control circuits of firing valves I2 I, HI and 32I extend from the lower end of the saturating reactor I8 to the center point 20 of three star-connected secondary windings I33, 233 and 333 of a control transformer 2I energized from the supply lines 3, 4 and 5. These three secondary windings correspond to the three firing valves I2I, HI and 32I, and the control circuit for each of these firing valves continues from the center point 20 through the corresponding secondary winding and a grid resistor I35, 235 and 335 to the associated control grid I31, 231 and 331.

The control circuits of firing valves I22, 222 and 322 continue from the center tap I1 of the saturating reactor I8 through the upper terminal thereof to the center point 22 of another set of three star-connected secondar windings I34, 234 and 334 of the control transformer 2I. These three secondary windings correspond to the three firing valves I22, 222 and 322. The control circuit of each of these three firing valves then continues from the center point 22 through the corresponding secondary winding and a grid resistor I36, 236 and 336 to the associated grid I38, 238 and 338.

It is apparent that the control circuit of each firing valve has impressed therein a negative biasing voltage through resistor I5 on which is superimposed a square-wave voltage through reactor I8, the square-wave voltage on the control circuits of firing valves I2I, HI and 32I being out of phase with the square-wave voltage for valves I 22, 222 and 322. In addition, each control circuit has impressed therein a voltage as developed across the corresponding winding of the secondaries of the control transformer 2I. It is to be noted that the voltage of each secondary winding of the control transformer 2I preferably is approximately 30 behind the phase voltage for the corresponding main valve. The

magnitudes of the biasin voltage, the squarewave voltage and the control transformer voltage are selected so that during the negative halfperiod of the square-wave voltage as referred to the control grid in a control circuit, the resultant voltage does not become more positive than the critical voltage which is necessary to render the corresponding firing valve conductive. On the other hand, during the positive half period of the square-wave voltage, the resultant voltage becomes more positive than the critical value to render the corresponding firing valve conductive at an instant approximately 30 behind the phase voltage for the corresponding main valve.

When a firing valve is rendered conductive, the corresponding firing capacitor discharges through the firing valve and the associated primary winding of the ignition transformer to supply an impulse of current through the ignition electrode of the corresponding main valve to render it conductive if a positive voltage exists across that main valve, that is, if at that instant the anode of the main valve is positive with respect to the cathode by an amount equal to 'or greater than the value necessary to permit initiation of and to maintain conduction of current therethrough.

The operation of the circuit of Fig. 1 proceeds as follows: While the square-wave voltage across the saturating reactor I8 is of such polarity that the lower terminal thereof is positive, firing valves I2I, 22I and 32I are rendered conductive in succession, each at an instant approximately 30 after the initiation of a positive half-period of the phase voltage supplied from the corresponding secondary windings I 00, 200 or 300 of the supply transformer 6 to the corresponding main valve IOI, 20I or 30I. It follows that the main valves IOI, 20I and 30I are then rendered conductive in succession with each becoming conductive approximately when the corresponding phase voltage for that main valve becomes more positive than the other phase voltages. Consequently, current flows in one direction from the outer ends of the secondary windings I00, 200 and 300 of the supply transformer 6 through main valves IOI, 20I and 30I and the primary winding I2 of the weldin transformer I I. This operation will be continued until the square-wave voltage across the saturating reactor I8 is changed.

When the polarity of the square-wave voltage is changed so that the upper terminal of the saturating reactor I8 becomes positive, one of the main valves -IOI, 20I and 30I will be conductive at that instant. Let it be assumed that the change in the polarity of the square-wave volt age occurs shortly after the main valve IOI is rendered conductive. Thereafter, at the instant determined by the phase relation of the secondary winding 334 of the control transformer 2I, the firing valve 322 is rendered conductive. Ignition current then flows through the ignition electrode 3I2 of the main valve 302, but this main valve does not become conductive because theanode 304 thereof is less positive than the anode I03 of the conductive main valve I I since anode 304 is connected directly to the cathode I of conductive main valve I0 I. v

Thereafter, the secondary winding 233 of t e control transformer 2|; tends to make the grid 231 of the firing valve 22I positive. However, since the polarity of the square-wave voltage is negative with respect to the grid 23! in the control circuit of this firing valv 22I, the grid 23! does not become sufficiently positive to render the firing valve 22I conductive.

The primary winding I2 represents an inductance in the circuit and magnetic energy is stored therein which is released as the phase voltage for conductive main valve IOI decreases and drops to zero. A capacitor 4| is connected across the primary winding I2 because of this fact. capacitor 4| obviously has a charge thereon of one polarity when the phase voltage for conductive valve IOI starts to drop. The capacitor then tends to supply current through the primary winding I2 until discharged and then absorbs the stored energy released by the primary winding, permitting the conductive main valve IM to become non-conductive when the phase voltage drops to zero or shortly thereafter as the phase voltage becomes negative. The capacitor also serves to improve the wave form of the primary winding current.

Approximately 30 degrees after the phase voltage for main valve IOI becomes negative, the secondary winding I34 of the control transformer 2| causes the firing valve I22 to become conductive. As a result, ignition current is supplied through the ignition electrode II2 of main valve I02. If the main valve IOI has become nonconductive by this time, main valve I02 becomes conductive to supply current in the opposite direction through primary winding I2 and main valves 202 and 302 become conductive in succession to continue such current until the polarity of the square-wave voltage changes again.

If the main valve IOI has not become nonconductiveby the time ignition current is supplied to main valve I02, that valve does not become conductive as a positive voltage does not exist thereacross. The secondary winding 333 of the control transformer 2| next tends to render' the firing valve 32I conductive, but is un successful since the square-wave voltage is negative with respect to the grid 331 of this firing valve.

By this time, the energy previously stored in the welding transformer has been consumed in charging capacitor M and maintaining the flow of current through the circuit including the conductive main valve IOI while the phase voltage across secondary winding I00 was negative so that main valve IOI becomes non-conductive. Thereafter, firing valves 222, 322 and I22 are rendered conductive in succession to render the main valves 202, 302 and I02 conductive in succession to effect a supply of current through the primary winding I2 of the welding transformer II in the opposite direction.

It then becomes apparent that, in effect, a single-phase current is supplied through the pri-- mary winding I2 of the welding transformer II at a frequency determined by the frequency of the output of the square-wave voltage generator I9. It is to be noted that the magnitude of the average current supplied through the primary winding I2 of the welding transformer II may be decreased by readjustment of the phase relationships of the voltages of the secondary windings of the control transformer 2I with respect to the supply votages to effect a delayed ignition of the main valves.

It is to be noted that if the inductance represented by the primary winding I2 is of such nature that when the square-wave voltage changes, the current through the primary winding I2 would drop to zero during the succeeding negative half-period of the phase voltage of the conductive main valve without the capacitor 4|, then the capacitor may be omitted. In either This case, it is to be noted the energy stored in the inductance represented by the primary winding I2 is not wasted but is returned to the supply transformer either in maintaining current fiow through the conductive valve while the phase voltage is negative, or, when the capacitor is used, in the subsequent discharge of the energy previously absorbed therein upon the release of the stored energy of the inductance, when the current supply through the primary winding is reversed.

Another way of taking care of the stored energy is shown in the circuit of Fig. 2 which is somewhat similar to that of Fig. 1 with the same reference numbers applied to corresponding elements. The load supply circuit of Fig. 2 differs from that of Fig. 1 in that the anodes I04, 204 and 304 of main valves I02, 202 and 302 are connected to the cathodes of valves IOI, I and 30I respectively through a paralleling reactor 24 instead of being directly connected as in Fig. l. The center tap 24a of the paralleling reactor 24 is then connected to the primary winding I2 of differs in the details of the control circuits of the firing valves I2l, I22, 22I, 222, 32I and The control circuits of each firing valve extends from the corresponding cathode ltl, I32, MI, 232, 33I and 332 through the resistor I5 across which the biasing potential appears to the center tap I! of the saturating reactor I8 which is energized from the square-wave voltage generator I9. 22I and 32I continue from the lower terminal of the saturating reactor I8 to the center point of three star-connected secondary windings MI, MI and MI of a first control transformer 26 of the peaking type energized from the supply lines 3, 4 and 5 through a first phase shifting unit 21. The three secondary windings MI, 24! and 3M correspond to the three firing valves I 2 I, 22I and 32I respectively and the control circuit of each of these firing valves extends from the center point 25 through the corresponding one of secondary windings I4I, 2M and MI and a corresponding one of secondary windings I43, 243 and 343 on a second control transformer 28 of the peaking type energized from the supply lines 3, 4 and 5 through a second phase shifting unit 29, and through a corresponding grid resistor I35, 235 and 335 to the corresponding grid I31, 23'! and 331. The control circuits for firing valves I22,

222 and 322 continue from the upper terminal of the saturating reactor I8 to the center of three other star-connected secondary windings I42, 242 and 342 of the first control transformer 26. These three secondary windings I42, 242 and 342 correspond to the three firing valves I22, 222 and 322 respectively and the control circuit of each of these firing valves extends from the center 30 through the corresponding one of secondary winding; I44, 244 and 344 on the second control transformer ,28 and then through a corresponding grid resistor I30, 236 and 336 to the corresponding grid I38, 238 and 338.

The secondary windings of the first and second control transformers 26 and 28 are so arranged and the phase shifting units are so adjusted that a first voltage impulse is impressed in the control circuit of each firing valve by the first con trol transformer making the grid thereof more positive at an instant approximately 30 after the initiation of a positive half-period of the The control circuits for firing valves I2I,

of the second voltage impulse.

correspondin phase voltage of the supply transformer for the corresponding main valve and a second voltage impulse is impressed in the control circuit of each firing valve by the second control transformer making the grid thereof more positive at an instant immediately after supply phase voltage for the corresponding main valve passes from positive to negative. In other words, the first control transformer provides a first voltage impulse for each firing valve tending to render it conductive approximately when the phase voltage for the corresponding main valve becomes more positive than the other phase voltages and the second control transformer provides a second voltage impulse for each firin valve tending to render it conductive approximately when the phase voltage for the corresponding main valve changes from positive to negative polarity.

It is to be noted that the magnitude of the first voltage impulse provided by the first control transformer 26 is considerably less than that of the second impulse provided by the second control transformer 28. The magnitudes of these impulses as well as of the biasing voltage across resistor I5 and the square-wave voltage from reactor I8 in each control circuit are selected so that while the square-wave voltage is in a positive half period with respect to the grid of the firing valve in a control circuit, the resultant voltage in the control circuit becomes more p0sitive than the critical value necessary to render the firing valve conductive at the instants of occurrence of both the first and second voltage impulses. On the other hand, when the squarewave voltage is in a negative half-period, the resultant voltage becomes more positive than the critical value only at the instant of occurrence The timing of the impulses is arranged to provide a smooth,

. faultless commutation or period of changeover of the supply of current through the primary winding I2 from one direction to the other as will appear hereinafter.

The operation of the circuit of Fig. 2 is as follows: While the lower terminal of the saturating reactor I8 is positive, let it be assumed that main valve I0i is first rendered conductive approximately 30 degrees after the corresponding phase voltage becomes positive with respect thereto to supply current in one direction through the primary winding 12. Thereafter, the phase voltage for main valve 30I passes from positive to negative and the second control transformer 23 provides a voltage impulse on the grid of firing valve 32I tending to render that firing valve conductive. If main valve 30I has been conductive just prior to main valve IOI it is probablethat capacitor SIS is not charged sufficiently to permit the firing valve 32I to become conductive. However, in any event it is apparent that the anode 303 of main valve 30I is less positive than the anode I03 of conductive main valve II]! at this time so that main valve 30I does not become conductive.

Approximately 30 degrees later, the first control transformer 20 provides a voltage impulse on the grid of firing valve 322. Since the squarewave voltage on reactor I8 is negative with respect to the grid of firing valve 322, the impulse is not suflicient to render the firing valve conductive.

As the phase voltage for main valve 202 thereafter passes from positive to negative the second control transformer impresses a voltage impulse on the grid of firing valve 222 sufficient to render it conductive even though the square-wave voltage on reactor 18 is negative with respect thereto. At this time it will be noted the sum of the voltages across secondary windings I and 200 is such that with main valve lill already conductive, a positive voltage exists across main valve 202 so that it also becomes conductive. Thus, at this time, main valves l0l and 202 are conductive simultaneously. In the absence of paralleling reactor 24 such a situation would establish a short circuit across the two secondary windings I00 and 200 and the current therethrough would soar resulting in loss of control and possibly injury to the main valves. The reactor 24 has a high impedance relative to that of any two secondary windings of the supply transformer and serves to prevent such a short circuiting and. to limit the current while both main valves I0! and 262 are conductive simultaneously. It is to be noted that the reactor 24 I may comprise two windings on separate cores or a single winding with a center tap as illustrated. In any case a high impedance is desired across the entire reactor to limit the current while two main valves are conductive but with a low impedance across one half of the reactor when but one main valve is conductive. To meet these conditions, we prefer a single winding with a center tap or two closely coupled windings on the same core.

The period of simultaneous conductivity of main valves lllland 202 lasts but a short while until the sum of the voltages across secondary windings I00 and 200 is insufiicient to maintain main valve 202 conductive. About the same time the first transformer 26 provides an impulse which is suflicient with the square-wave voltage on reactor I8 to render firing valve 22l conductive and so main valve 20l becomes conductive to take over from valve 101 the supply of current in the same direction through the primary winding l2.

Thereafter impulses are provided by the second control transformer on firing valve l2l for main valve and then by the second control transformer on firing valve I22 for main valve [02. However, main valves [0! and I02 are not rendered conductive by these impulses for the reasons set forth in connection with main valves 30l and 302 while main valve [0! was conductive.

Next, the second control transformer 28 impresses a voltage impulse on the grid of firing valve 322 as the phase voltage for the corresponding main valve 302 passes from positive to negative. Main valve 302 then becomes conductive while main valve l is still conductive. Shortly thereafter main valve 30l is rendered conductive to take over from main valve 201 the supply of current in the same direction through the primary winding l2.

Subsequently main valve I02 becomes c0nductive while main valve 30l is still conductive and then main valve l0l takes over from valve 30l. Thus, the main valves ml, 20! and 30! continue to become conductive in succession to supply current in one direction through the primary winding l2 until the polarity of the square-wave voltage on reactor l8 changes.

When the square-wave voltage on reactor I8 changes, the main valves which previously 0perated as rectifiers to supply current to the primary winding l2 of the welding transformer,

that is, valves [0], 20I and 30l in the situation just discussed, now operate as inverters until the current through the primary winding drops substantially to zero. This may be appreciated from a concrete example where it is assumed that the polarity oi the square-wave voltage on the saturating reactor I8 is changed shortly alter the main valve 101 is rendered conductive. In following the order of peaKed voltage impulses impressed in the control circuits from both the first and second control transiormers 20' and 28, it is seen that the second control transiormer 28 first impresses an impulse on the grid of I11- ing valve SZI for main valve 30l. However, the anode 303 of main valve 30! is considerably less positive at this instant than the anode I03 of conductive valve i0l so that main valve 30! does not become conductive.

The first control transformer 26 next provides a peaked impulse to render firing valve 322 conductive. However, current is still flowing through valve H from the secondary winding I00 so that the anode 304 of main valve 302 is less positive than that of main valve WI and tinues toconduct. The succeeding impulse supplied in the control circuit of firing valve 22l by the first control transformer 26 is insufficient to render that firing valve conductive as the lower terminal of the reactor I8 is now negative.

The firing valve I2l next receives an impulse from the second control transformer 28, but it is noted that capacitor H9 is not charged sufficiently for firing valve l2l to conduct and the corresponding main valve l0l is already conductive anyhow. Thereafter, the voltage impulse of the first control transformer 26 renders firing valve I22 conductive. However, main valve 10! is still conducting current supplied by the release of energy previously stored in the primary winding l2 of the welding transformer. Since the cathode of main valve I02 corresponding to firing valve I22 is connected to the anode of main valve l0l, main valve I02 does not become conductive.

The subsequent impulse of the second control transformer 28 in the control circuit of the firing valve 322 does not render main valve 302 conductive as the voltage thereacross is negative at that time. The succeeding impulse in the control circuit of firing valve 32! from the first control transformer does not render that firing valve conductive because the square-wave voltage is negative with respect thereto. As the phase voltage corresponding to mainvalve 20| is passing from positive to negative the second control transformer tends to render that valve conductive. At that same instant, the phase voltage corresponding to conductive valve I0! is highly negative, the valve IOI remaining conductive because of the voltage generated across the primary winding l2 by the release of previously stored energy. Consequently, valve 20I has a more positive voltage thereacross and is rendered conductive while valve l0l becomes non-conductive.

This procedure continues with main valve 20] acting as an inverter to be followed by main valve 30| acting as an inverter and so on until the current generated by the primary winding i2 of the transformer 6 drops to zero. Thereafter, valves I02, 202 and 302 are rendered conductive as rectifiers in succession at the instant when each corresponding phase voltage becomes more positive than the other phase voltages to supply current in the opposite direction through the primary winding l2. Of course main valves 20!, Sill and NH are rendered conductive for a short time near the end of each period of conduction of main valves I02, 202 and 302, respectively. When the square-wave voltage is subsequently changed, valves I02, 202 and 302 act as inverters until the current in primary winding l2 drops to zero and valves I01, 21H and 30! take over to supply current in the first direction again through the primary winding. Thus, current of a low frequency relative to that of the supply voltage is supplied through the primary winding 112 with the frequency determined by the frequency of the square-wave voltage on reactor l8.

While we have shown and described specific embodiments of our invention, we are aware that many other modifications thereof may be made without departing from the spirit of the invention. We do not intend, therefore, to limit our invention to the specific embodiments disclosed.

We claim as our invention:

1. Apparatus for supplying power from a source of polyphase alternating voltage to a single phase load, comprising, in combination, a pair of unidirectional electric discharge valves for each phase of said source, each pair of valves being connected in inverse parallel-circuit relation with each other and in series with the corresponding phase of said source and said load, whereby load current in one direction may be conducted only through a first valve of each pair and load current in the opposite direction may be conducted only through a second valve of each pair, timing means for causing a control current to fiow in one direction during a first time interval and to flow in the other direction during a second time interval, and. control electrodes in said valves and responsive to said timing means in accordance with the direction of said control current flow caused by said timing means for causing said first valves to be conductive to effect a load current in said one direction during a first time period and said second valves to be conductive to effect a load current in the opposite direction during a second time period following said first time period, the length of said first and second time periods being determined by the length of said first and second time intervals, respectively.

2. Apparatus for supplying power from a source of polyphase alternating voltage to a single phase load, comprising, in combination, a pair of unidirectional electric discharge valves for each phase of said source, each pair of valves being connected in inverse parallel-circuit relation with each other and in series with the corresponding phase of said source and said load, whereby load current in one direction may be conducted only through a first valve of each pair and load current in the opposite direction may be conducted only through a second valve of each pair, timing means for producing a square-wave voltage of a frequency substantially lower than that of said alternating voltage, and control electrodes in said valves and responsive to said square-wave voltage for causing said first valves to be conductive to effect a load current in said one direction during a first time interval and said second valves to be conductive to effect a load current in said opposite direction during a second time interval following said first intervals, the lengths of said intervals being determined by the frequency of said square-wave voltage.

3. Apparatus for supplying power from a source of polyphase alternating voltage to a single phase load, comprising, in combination, a pair of electric discharge valves of the arc-like type for each phase of said source, each pair of valves being connected in inverse parallelcircuit relation with each other and in series with the corresponding phase of said source and said load, whereby load current in one direction may be conducted only through a first valve of each pair and load current in the opposite direction may be conducted only through a second valve of each pair, an impedance means timing means for impressing a voltage across said impedance means which varies at the end of successive time intervals, and control electrodes in said valves and responsive to said voltage for rendering each of said first valves conductive in each half-period of the corresponding phase voltage in which a positive voltage exists across that first valve during alternate ones of said intervals and each of said second valves conductive in each half period of the corresponding phase voltage in which a positive voltage exists across that second valve during the other ones or said intervals.

4. Apparatus for supplying power from a source of polyphase alternating voltage to a single phase load, comprising, in combination, a, pair of electric discharge valve of the arc-like type for each phase of said source, each pair of valves being connected in inverse parallel-circuit relation with each other and in series with the corresponding phase of said source and said load, whereby load current in one direction may be conducted only through a, first valve of each pair and load current in the opposite direction may be conducted only through a second valve of each pair, timing means for producing a square-wave voltage of a frequency substantially lower than that of said alternating voltage, and control electrodes in said valves and responsive to said square-wave voltage, said control means being responsive to half periods of said square-wave voltage of one polarity to render each Of said first valves conductive in each half-period of the corresponding phase voltage in which a positive voltage exists across that first valve and responsive to half-periods of the opposite polarity to render each of said second valves conductive in each half-period of the corresponding phase voltage in which a positive voltage exists across that second valve.

5. Apparatus for supplying power from a source of polyphase alternating voltage to a single phase inductive load, comprising, in combination, a pair of electric discharge valves of the arc-like type for each phase of said source, each pair of valves being connected in inverse parallel-circuit relation with each other and in series with the corresponding phase of said source and said load, whereby load current in one direction may be conducted only through a first valve of each pair and load current in the opposite direction may be conducted only through a second valve of each pair, an impedance means, timing means for impressing a voltage across said impedance means which varies at the end of successive time intervals, control electrodes in said valves and responsive to said voltage for rendering each of said first valves conductive in each half-period of the corresponding phase voltage in which a positive voltage exists across that first valve during alternate ones of said intervals and each of said second valves conductive in each half-period of the corresponding phase voltage in which a positive voltage exists across that second valve during the other ones of said intervals, and a capacitor connected in shunt across said load to absorb stored energy released by the load at the end of each interval.

6. Apparatus for supplying power from polyphase alternating voltage supply lines to a single phase inductive load, comprising, in combination, a polyphase transformer adapted to be energized from said supply lines and having a plurality of star-connected secondary windings, a first discharge valve of the arc-like type for each secondary winding having an anode and cathode with the anode connected to the outer end of the corresponding secondary winding, a second discharge valve of the arc-like type for each secondary winding having an anode and cathode with the cathode connected to the anode of the first valve for the corresponding secondary Winding, an impedance member having two sections connected between the cathodes of said first valves and the anodes of the second valves, means adapted to connect said load from the center of said star-connected secondary windings to a point intermediate said two impedance sections, and control means for rendering conductive each of said first valves only in each positive half-period and each of said second valves only in each negative half-period of voltage across the correspondin secondary winding during a first predetermined time interval and each of said first valves only in each negative half-period and each of said second valves only in each positive halfperiod of Voltage across the corresponding secondary winding in which the voltage across the valve is positive during a second immediately succeeding predetermined time interval.

'7. Apparatus for supplying power from polyphase alternating voltage supply lines to a single phase inductive load, comprising, in combination, a polyphase transformer adapted to be energized from said supply lines and having a pluralit of star-connected secondary windings, a first discharge valve of the arc-like type for each secondary winding having an anode and cathode with the anode connected to the outer end of the corresponding secondary winding, a second discharge valve of the arc-like type for each secondary winding having an anode and cathode with the cathode connected to the anode of the first valve for the corresponding secondary winding, a reactor connected between the cathodes of said first valves and the anodes of the second valves, means adapted to connect said load from the center of said star-connected secondary windings to an intermediate point on said reactor, and control means for rendering conductive each of said first valves only in each positive half-period and each of said second valves only in each negative half-period of voltage across the corresponding secondary winding during a first predetermined time interval and each of said first valves only in each negative halfperiod and each of said second valves only in each positive half-period of voltage across the corresponding secondary winding in which the voltage across the valve is positive during a second immediately succeeding predetermined time interval.

8. Apparatus for supplyin power from polyphase alternating voltage supply lines to a single phase inductive load, comprising, in combination, a polyphase transformer adapted to be energized from said supply lines and having a plurality of star-connected secondary windings, a first discharge valve of the arc-like type for each secondary winding having an anode and cathode with the anode connected to the outer end of the corresponding secondary winding, a second discharge valve of the arc-like type for each secondary winding having an anode and cathode with the cathode connected to the anode of the first valve for the corresponding secondary winding, an iron core reactor having two sections on a single core connected between the cathodes of said first valves and the anodes of the second valves, means adapted to connect said load from the center of said star-connected secondary windings to a point intermediate said two sections, and control means for rendering conductive each of said first valves only in each positive halfperiod and each of said second valves only in each negative half-period of voltage across the corresponding secondary Winding during a first predetermined time interval and each of said first valves only in each negative half-period and each of said second valves only in each positive halfperiod of voltage across the corresponding secondary winding in which the voltage across the valve is positive durin a second immediately succeeding predetermined time interval.

9. Apparatus for supplying power from polyphase alternating voltage supply lines to a single phase inductive load, comprising in combination, a polyphase transformer adapted to be energized from said supply lines and having a plurality of star-connected secondary windings, a first discharge valve of the arc-like type for each secondary Winding having an anode and cathode with the anode connected to the outer end of the corresponding secondary winding, a second discharge valve of the arc-like type for each secondary winding havin an anode and cathode with the cathode connected to the anode of the first valve for the corresponding secondary Winding, animpedance member having two sections connected between the cathodes of said first valves and the anodes of the second valves, means adapted to connect said load from the center of said star-connected secondary windings to a point intermediate said two impedance sections, timing means for indicating a series of successive time intervals, and control means responsive to said timing means for rendering conductive each of said first valves only in each positive half-period and each of said second valves only in each negative half-period of voltage across the corresponding secondary winding in Which the voltage across the valve is positive during alternate ones of said time intervals and rendering conductive each of said first valves only in each negative half-period and each of said second valves only in each positive half-period of voltage across the corresponding secondary winding in which the voltage across the valve is positive during the other ones of said intervals.

10. Apparatus for supplying power from a source of polyphase alternating voltage to a single phase load, comprising, in combination, a pair of ignitrons for each phase of said source, the ignitrons of each pair being connected in inverse parallel-circuit relation with each other and in series with the corresponding phase of said source and said load, whereby load current in one direction may be conducted only through a first ignitron of each pair and load current in the opposite direction may be conducted only through a second ignltron of each pair, an auxiliary valve associated with each of said ignitrons and a control means connected to and individual to each of said auxiliary valves, each of said control means operating independently of the other of said control means for causing said auxiliary valves to cause said first ignitrons to be conductive in succession to effect a load current in said one direction during a predetermined first time interval and said second ignitrons to be conductive in succession to effect a load current in said opposite direction during a predetermined second time interval follow'ng said first interval, said time intervals being determinable independently of the period of said source and being of longer duration than a period of said source.

11. Apparatus for supplying power from a source of polyphase alternating voltage to a single phase load, comprisin in combination, a pair of main electric discharge valves of the arc-like type for each phase of said source, the valves of each pair being connected in inverse parallel circuit relation with each other and in series with the corresponding phase of said source and said load, whereby load current in one direction may be conducted only through a first valve of each pair and load current in the opposite direction may be conducted only through a second valve of each pair, an auxiliary valve associated with each of said main valves and a control means connected to and individual to each of said auxiliary valves for controlling said auxiliary valve, each of said control means operating independently of the other of said control means for causing said auxiliary valves to render said first main valves conductive in succession to efiect a load current in said one direction during a predetermined first time interval and said second main valves conductive in succession to effect a load current in said opposite directon during a predetermined 16 second time interval following said first interval, said time intervals being determinable independently of the period of said source and being of longer duration than a period of said source.

12. Apparatus for supplying power from a source of polyphase alternatin voltage to a single phase load, comprising, in combination, a pair of ignitrons for each phase of said source, the ignitrons of each pair being connected in inverse parallel-circuit relation with each other and in series with the corresponding phase of said source and said load, whereby load current in one direction may be conducted only through a first ignitron of each pair and load current in the opposite direction may be conducted only through a second ignitron of each pair, an auxiliary valve associated with each of said ignitrons, and a purely electrical control means connected'to and individual to each of said auxiliary valves for controlling said auxiliary valves, each of said control means operating independently of the other of said control means to cause said auxiliary valves to cause said first ignitrons to be conductive in succession to effect a load current in said one direction during a predetermined first time interval and said second ignitrons to be conductive in succession to effect a load current in said opposite direction during a predetermined second time interval following said first interval, said time intervals being determinable independently of the period of said source and being of longer duration than a period of said source.

JOHN L. BOYER. WILLIAM E. PAKALA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,954,661 Alexanderson et a1. Apr. 10, 1934 1,957,230 Sabbah et a1 May 1, 1934 2,319,524 Undy May 18, 1943 2,337,982 Rodgers Dec. 28, 1943 2,356,859 Leathers Aug. 29, 1944 2,385,214 Livingston Sept. 18, 1945 

